Modern wind turbine blades are typically moulded from composite materials such as glass-fibre reinforced plastics (GFRP). The blades extend longitudinally from a generally circular root end towards a tip end.
FIG. 1 shows the root end 10 of a typical wind turbine blade 12. A plurality of metal root inserts 14 are embedded in the root end 10 of the blade 12. The root inserts 14 are substantially tubular and of circular cross section. The root inserts 14 extend axially into the root end of the blade, as shown by way of example by the dashed lines 14a. Each insert 14 defines a longitudinal bore 16, which has an internal thread. The inserts 14 are equally spaced around the circumference of the root end 10. The inserts 14 each have a flat annular end surface 18 surrounding and defining the opening 20 of the bore 16. The flat end surfaces 18 of the inserts 14 lie flush with the annular root end surface 22 of the blade 12.
The root inserts are used for connecting the blade to a pitch bearing mounted to the hub when the blade is installed on a wind turbine. Stud bolts are used to secure the blade to the pitch bearing, with the bolt shafts being received within the threaded bores of the inserts. The inserts are also used during manufacture of the blade to support the blade during the moulding process. In this respect, the root end of the blade is secured to a mount located at one end of the blade mould by means of studs that protrude from the mount into the bores. Copper grease is applied to the studs and to the bores to enable the root end of the blade to be released from the studs when the blade is removed from the mould.
Once the blade has been removed from the mould, the root end of the blade is machined to provide a flat level surface, which forms the interface with the pitch bearing. An orbital milling machine is used for this purpose, and typically one or two millimeters of material is removed from the root end to form the required flat surface.
A problem associated with the milling process is that the swarf produced during milling tends to accumulate inside the bores of the root inserts. This problem is exacerbated by the presence of the copper grease inside the bores to which the swarf sticks, thus making it very difficult to remove the swarf from the bores after the milling process.
Prior to the blades leaving the factory, each root insert undergoes a thorough cleaning process to remove any debris or swarf from the bores. As each blade includes a large number of root inserts, this cleaning process is both time consuming and expensive. It is also difficult to verify the cleanliness of the bores after the cleaning process, and so there remains the possibility that small amounts of debris may remain in one or more of the bores even after the cleaning process, or worse that one of the bores may accidentally have been omitted altogether from the cleaning process.
Once the bores have been cleaned, a tubular bung is inserted into each bore. The bung has a threaded outer surface and is screwed into the bore. The bung includes an integral end cap having a circumferential flange which overlies the machined annular surface of the root insert when the bung is inserted into the bore. A washer is typically provided between the flange and the annular surface. The bung prevents debris from entering the bore during storage and transportation of the blade, whilst the washer and flange serve to protect the annular surface of the root insert. The bung is removed immediately prior to installation of the blade.
Connecting the blades to the hub is a difficult and expensive process at the best of times. However, if the cleaning process described above has not been sufficiently thorough then it is possible that debris may still be present inside the bores at the time of installation. The debris may prevent the stud bolt from fastening correctly inside the bore or in some cases it may cause the bolt to become stuck inside the bore. The presence of debris inside the bores at the time of installation is therefore a serious problem and will delay, and in some cases prevent, the installation of the blade.
The present invention aims to overcome one or more of the problems described above.